I think I'll go ahead and change the meaning of '-' and '+' when used with %C. Hopefully this won't affect too many people. Here is the proposed update to the documentation:
A special feature allows the copy number to be incremented for
each processed file by using %C (upper case) instead of %c. This
allows a sequential number to be added to output file names, even
if the names are different. For %C, a copy number of zero is not
omitted as it is with %c. A leading '-' causes the number to be
reset at the start of each new directory, and '+' has no effect.
[...]
This will be implemented in ExifTool 11.24 unless I hear an objection.
- Phil